(a) Field of the Invention
The present invention relates to a mobile station intermittently receiving a signal and, more particularly, to a mobile station intermittently receiving a signal from a base station and periodically operating for a specific task.
(b) Description of the Related Art
A mobile station used in a GSM (global system for mobile communications) or IMT-2000 (international mobile telecommunications 2000) has a mobile terminal and an IC card inserted therein for storing information of the subscriber, such as a telecommunication number. In the GSM, for example, the IC card is generally called smart card or subscriber identity module (SIM) card.
Since the mobile station uses the information of the subscriber stored in the IC card for originating a call, the mobile station cannot successfully originate the call without the IC card. Thus, the mobile station inserting therein an IC card generally detects the presence/absence of the IC card (hereinafter referred to merely as “IC card detection”) periodically at a specified interval in a waiting state of the mobile station. The technique for the IC card detection is described in JP-A-6-296161, -11-191797 and -2001-238260, for example.
The IC card detection is performed for the purpose of prohibiting the mobile station from originating a call without the IC card inserted therein. This purpose necessitates an early IC card detection in the mobile station after the IC card is removed therefrom. Thus, it is requested for the mobile station to periodically execute the IC card detection within a short time interval.
In the meanwhile, the mobile station intermittently operates for reception of a signal via a paging channel from the base station (hereinafter referred to merely as “signal reception”) in the waiting state for the purpose of detecting an incoming call. This timing is performed in synchrony with the timing determined by the base station, and thus may be changed if the mobile station moves from a base station to another base station for an external handover.
FIG. 7 shows a timing chart showing the interval of the IC card detection (ICD) and the interval of signal reception processing (SRP) in a conventional mobile terminal. In FIG. 7, for example, the IC card detection (ICD) continues for around 30 to 40 milliseconds and the interval (first interval) of the IC card detection is around 30 seconds, whereas the signal reception processing (SRP) continues for around 20 to 100 milliseconds and the interval (second interval) of the signal reception processing is around 2 to 4 seconds. In other words, the IC card detection and the signal reception processing are performed in asynchronous operations. Thus, the CPU installed in the conventional mobile station manages the first interval and the second interval independently of each other.
If the mobile station stays in the waiting state for a specified time interval, the mobile station generally assumes a sleep mode (or idle mode) to thereby reduce the power dissipation. In such a circumstance, since the CPU manages the first interval and the second interval independently of each other to exit from the sleep mode into an active mode, a longer time length is needed for the mode shift, thereby increasing the power dissipation of the mobile station. In particular, since the clock signal stopped in the sleep mode must be generated in the active mode, the mode shift from the sleep mode to the active mode takes a longer time due to the necessity of assuring stabilization of the clock signal.